The transmission of high-quality sound by frequency modulation (FM) or phase modulation (PM) originally included a relatively simple, straightforward signal path. The audio signal was received from a source and served to modulate a carrier frequency signal, which was transmitted by wire or through the air to a receiver which demodulated the signal to produce a relatively clean audio signal. However, as the number of signals increased, efficient channel allocation became a significant problem. A typical solution is to translate or convert the original carrier frequency of the original signal to a new frequency closely spaced with other signals, providing efficient transfer of many channels of data through a selected media of finite bandwidth. Frequently, the process of signal translation is applied several times. For instance, a local television signal may be translated to a microwave link frequency, which is then translated to a frequency to be received by a satellite. The satellite again converts the signal frequency, which is then reconverted back down to television VHF, UHF, or CATV signal, to be finally received by the local subscriber. In the example provided, the signal is combined or mixed with several local oscillator signals going upward in frequency, and again with several local oscillator signals to be converted down to baseband, each having their own phase jitter and frequency shift signal components. For systems using frequency or phase modulation, every source of phase jitter and frequency shift signal components caused by frequency translation and component aging further degrades the recovery of the modulated signal, resulting in a significant reduction in the final channel signal-to-noise ratio and dynamic range capability, with the phase jitter and frequency shift signals appearing as audible noise in the received signal. This signal degradation due to noise is particularly noticeable for limited frequency modulation deviation (less than 40 KHz deviation).
The audio signals, often of poor audio quality for the reason mentioned above, are considered noticeably less listenable in comparison to the high-quality FM, stereo signals when reproduced through a common high-fidelity system. Moreover, the monaural nature of most television audio signals makes the reproduction of television sound appear even more inferior. As a result, the usability of the audio portion of television signals, as well as any signals rebroadcast at a different frequency, is severely limited.